FIGS. 1 and 2, to which reference is now made, show a cutting device of the blade and counterblade type normally used, notably for cutting photosensitive products.
This conventional cutting device comprises principally blades 1 and counter-blades 2 mounted respectively on two parallel shafts 3 and 4 supported at their respective ends by suitable support means (not shown). In the embodiment shown, the blades and counter-blades are of the lip type, that is to say they have, on their periphery, a surface substantially parallel to the axes of the shafts. The blades and counter-blades are held at equal axial distances from each other on each of their respective shafts by means of circular rings 5, 6 mounted on each of the shafts 3, 4. The distance between the shafts is adjusted so as to allow a certain height of engagement h between the blades and counter-blades in order to obtain a satisfactory quality of cutting of the sheet of material passing between the two cutting shafts. In addition, in order to provide an even quality of cutting, this height of engagement must not vary too much; the same applies to the uniformity of the width of strips produced. Typically the value of h is of the order of 0.6 mm.
Thus this height of engagement h must be substantially constant over the entire length of the shafts, which are supported at their ends by suitable means of the ball-bearing type well known in the art. Such an arrangement does not pose any problem for short, lightweight shafts. On the other hand, for long, heavier shafts it is necessary to provide, at one or more positions over the length of the cutting shafts, holding devices designed to limit the maximum distance between these shafts and thus to guarantee, at any point on the cutting device, a minimum penetration value h between the cutting edges of the blades and counter-blades, whatever the value of the tension of the sheet running between the shafts. In fact, the variations in tension of the sheet, due particularly to variations in thickness of the sheet as a fold or join passes, cause a relative separation of the shafts which may take out the engagement height between the blades and counter-blades leading to a phenomenon commonly referred to as "cutting jump". In such circumstances, in fact, the blades are no longer in cutting engagement with the counter-blades, thus interrupting the cutting of the sheet into longitudinal strips and damaging the cutting tools.
To resolve this problem, it is known that cylindrical support members, 7, 8, 9, 10 can be used, carried by the frame of the cutting machine and designed to be brought into contact with the shafts, either at the circular rings 5 (as shown for the shaft 3 carrying the blades) or at the peripheral surface of the counter-blades (as shown for the shaft 4 carrying the counter-blades). These cylindrical members are generally disposed in pairs (7, 8; 9, 10) for each of the shafts 3, 4 on each side of the plane passing through the centers of the two shafts. According to known techniques, bearings or cylindrical rings made from hardened steel are used, disposed so as to be continuously in contact with the shafts.
Such devices have the advantage of precisely limiting the maximum distance between the shafts because of the rigidity of the metal-to-metal contact of the cylindrical support members and the bearing surfaces on the cutting shafts. The metal-to-metal contact, however, constitutes a major drawback because of the damage to the surface of the shafts (particularly disadvantageous when the contact takes place on the blades or counter-blades) and therefore because of the damage to the sheet passing in contact with the cutting shafts. Such a contact also generates harmful vibration. According to other known devices, the problem of the metal-to-metal contact is resolved by covering the cylindrical members with a layer of flexible material such as polyurethane. The drawback of such a solution lies in the fact that, because of the thickness of the layer of flexible material (of the order of 2 mm), it does not make it possible to limit sufficiently the relative distance between the shafts when there is any abrupt change in tension in the sheet to be cut. Such a solution does not therefore avoid the problem of the cutting jumps mentioned above.